adds command line switch --with-numpydoc as suggested in issue #26

To add the numpydoc module the placeholder ${numpydoc} is added to
the extensions variable in sphinx_conf.template.
It is set to an empty string by default and to
", 'numpydoc'" if --with-numpydoc is specified.

- added tests for cases with and without numpydoc
- added sphinx as dependency for test runs
- updated documentation
Sorry, had not installed autopep8 in the virtualenv I was working in. I'm so used to it that I don't see blank lines as a problem since they normally disappear upon saving the file. Must add it to my standard packages.